Xal'atath(as far as we know) isn't an Old God so it was easier to trap her. But it was a powerful Old God artifact which is why it was able to pierce N'Ztoh's carapace. Sure he had plans but that doesn't matter if he's killed before he can fulfill those plans.
So a Void expansion would be the only way we see N'Zoth again.
I can think of a few, while deleterious to the lore, that could at least halt or limit the damage Teldrassil did to the game as a whole:
- Retcon it so that Sylvanas entered into a deal with N'Zoth beforehand, and N'Zoth subsequently influenced the Horde and artificially inflamed their hatreds in a general sense.
- Explore those who actually assisted with firing the siege weapons at Teldrassil and expose them as Sylvanas' cultists and perhaps even agents of the Jailer in the physical world.
- Retroactively institute a "fog of war" narrative to the Horde, making it so that what happened at Darkshore is not and was never common knowledge to the rank and file, painting it as if many in the Horde felt it was a false flag, so to speak, or that it was an "accident" that the Alliance are trying to blame on the Horde (a redux of the Wrathgate if you will).
The developers are already doing a bit of damage control i.e. Teldrassil with the events in Ardenweald during Shadowlands. They're not doing what I feared they might do, fortunately; but it is still a pretty obvious attempt to minimize the damage.
"We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see." ― Tom Stoppard, Rosencrantz and Guildenstern are Dead
I would not say, that they waste their villains. Its more, that they are just "kinda" bad at writing villians with a start, mid and ending. They set villians up, but they have not planned out when they are gonna be used or when they are gonna die.
That makes it so, that most villains role in world feels either rushed or ignored. They often die to either end an expansion or to make room for another villain. That kind of character writing is insanely hard to get right, since you have to write for both long and short term at the same time. Every event with the character has to be both a middle section and an end point at the same time.
So yeah, they are not wasting them, there is just a really bad infrastructure for villians in WoWs story.
May the lore be great and the stories interesting. A game without a story, is a game without a soul. Value the lore and it will reward you with fun!
Don't let yourself be satisfied with what you expect and what you seem as obvious. Ask for something good, surprising and better. Your own standards ends up being other peoples standard.
"We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see." ― Tom Stoppard, Rosencrantz and Guildenstern are Dead
Because a bad guy needs to be insane, corrupted or irredeemably evil for us to fight them.
There can be no moral ambiguity if someone is a loot pinata. Our collective conscience must be clean.
Narratively speaking, you could make it a one-time event through storytelling element - such as Tyrande expelling the power of the Night Warrior (along with perhaps her own life) to resurrect them and essentially undo what had been done. I don't think this would be a good way to go about it by any means, but it could be done in a manner that doesn't itself open a huge plot hole.
"We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see." ― Tom Stoppard, Rosencrantz and Guildenstern are Dead
I predict that Sargeras, Azshara and Nzoth will return for the great end.
Firstly, there are rumours that Xal'atath may in fact be an Old God herself, but that's beside the point. The whole plot thread with N'Zoth intentionally emptying it, delivering it to Sylvanas so that she could give it to Azshara who somehow was under the impression that it would be able to pierce N'Zoth. All of that only makes sense if N'Zoth had some sort of plan for it, and the most obvious way to use a Blade which we know stored something which might be an Old God is to store another one.
Grom isn't dead, he died "off screen" but take my word for it that we'll see him chained up.. again, we'll then free him and at a later point he will heroically sacrifice himself in [insert plot here]
"Theories abound as to the nature of its creation: the more outlandish claim that the blade is the remains of a forgotten Old God who was consumed by its kin, while other theories state it is the claw of Y'Shaarj itself."
This is literally the only time that it is said that Xala'atath might be an Old God. Notice how it says that it's an outlandish theory. So no. In that context, she absolutely isn't an Old God.
Seems a strange detail to put in-game. Regardless, whether Xal'atath is an Old God or not is irrelevant. There's no reason that N'Zoth couldn't store a part of his soul or something inside the Blade. For what other reason would he empty it and deliver it to Sylvanas?